1. Field of the Invention
The present invention relates generally to a voltage drive circuit for driving a stepping motor for a laser scanning motor in a laser bar code reader assembly, and more particularly pertains to a voltage drive circuit as described for driving a stepping motor in a manner in which the position of the rotor precisely follows an input voltage signal driving the motor by utilization of multiple feedback loops and microstepping techniques.
2. Discussion of the Prior Art
The increased use of bar code symbols to identify products, particularly in retail businesses, has resulted in the development of various bar code reading systems. Many users of bar code readers require portable hand-held scanners which place a premium on small size, lightweight and low power consumption requirements for the devices. One such system is a laser scanning bar code reading system as described in U.S. Pat. No. 4,496,831, commonly assigned to the same assignee as the present patent application.
The laser scanning system disclosed in U.S. Pat. No. 4,496,831 includes a portable hand-held scanning head which may be embodied in various shapes but preferably has a gun shaped housing made of lightweight plastic. A handle and barrel portion are provided to house the various components of the scanning head therein. Within the barrel portion are mounted a miniature light source, a miniature optic train including focusing lenses and a scanning system for directing light from the light source across a bar code symbol, and miniature sensing means for detecting reflected light from the bar code symbol being scanned.
The miniature light source can comprise a laser tube such as a coaxial helium neon laser tube, or preferably a semiconductor laser diode which is considerably smaller and lighter than a laser tube, thus reducing the required size and weight of the scanning head and making the scanning head easier to handle and more maneuverable. Light generated by the light source passes through the optic train which directs the beam onto the scanning system which is mounted within the barrel portion of the scanning head. The scanning system sweeps the laser beam across the bar code symbol and comprises at least one scanning stepping motor for sweeping the beam lengthwise across the symbol, and may comprise two motors wherein the second motor sweeps the beam widthwise across the symbol. Light reflecting mirrors are mounted on the motor shaft or shafts to direct the beam through the outlet port to the symbol.
A sensing circuit then detects and processes the light reflected from the symbol and generally comprises a photodetector element such as a semiconductor photodiode. The user positions the hand-held unit so the scan pattern traverses the symbol to be read, and the photodetector element produces serial electrical signals to be processed for identifying the bar code. A signal processing circuit for a bar code produces a signal which is directed to a bar pattern decoder circuit for decoding the bar pattern.
The reader unit can have a movable trigger employed to allow the user to activate the light beam and detector circuitry when pointed at the symbol to be read, thereby conserving battery life if the unit is self-powered. The lightweight plastic housing contains the laser light source, the detector, the optics, signal processing circuitry, a CPU and a battery. The reader is designed to be aimed at a bar code symbol by the user from a position where the reader is spaced from the symbol, i.e., not touching the symbol or moving across the symbol. Typically, this type of hand-held bar code reader is specified to operate in the range of perhaps several inches.
The prior art has generally utilized a current drive circuit for driving the stepping motor in a laser bar code reader scanner. In such prior art current drive circuits, the connections to the AC and DC windings of the stepping motor are reversed from those of a voltage drive circuit as illustrated in FIG. 1 herein and as explained hereinbelow. Namely, the drive voltage potential is introduced to the center tap of the AC winding 14, and the opposite ends of each phase of the AC winding are grounded, and the ground and potential sides of the DC winding 18 are also reversed, in a manner opposite to that shown in FIG. 1 herein. In a prior art current drive circuit, the current through the AC winding 14 is detected in a current feedback loop to ensure that the current through the AC winding follows an input current. In contrast thereto, in a voltage drive circuit as taught by the present invention, the voltage through the AC winding 14 is detected in a feedback loop to ensure that the voltage across the winding follows the input voltage.
The advantages of the prior art current drive circuits include a reduction of the phase delay of the motor winding inductance, which allows a start of scan (SOS) timing signal to be generated directly from an oscillator signal. Moreover, the temperature rise in the motor winding is compensated, and Vbat-0.7 v can be supplied to the motor windings. However, prior art current drive circuits include a number of disadvantages including an exaggeration of the resonance of the motor. The system is high q (q is used herein as the classical measure of the quality or resonance as applied to resonant systems, both electrical and mechanical) and the frequency is sensitive to drive amplitude, motor efficiency and mirror mass. The arrangement is sensitive to motor efficiency, to load mass and inertia, to bearing friction and damping, and to drive frequency shifts. Moreover, the nonlinear restoring force of the DC winding causes "blooming." In summary, the system needs to be mechanically tuned to work properly.
The advantages and disadvantages of a current drive circuit make it suitable for driving a stepping motor with sleeve bearings, which provide friction and damping for the motor. However, a current drive circuit is not suitable for driving a stepping motor with ball bearings, which provide negligible friction and damping for the motor, and also advantageously have a very long service life. The high damping and low q provided by the voltage drive circuit of the present invention make it much more suitable for driving a stepping motor with ball bearings.